The present invention relates to an MR imaging method and an MRI system.
More particularly, the present invention is concerned with an MR imaging method and an MRI system that clearly separates water in an object from fat therein for a short period of time so as to construct an image.
MR imaging methods and MRI systems are demanded to construct an accurate image for a short period of time.
Scanning an object whose spins are excited in the steady-state free precession (SSFP) method has advantage of producing an intense signal, which represents a high contrast, for a short scan time. On the other hand, the SSFP method confronts the problems that band artifact occurs and that it is hard to separate water from fat due to induction of an intense signal by fat.
As a technique for suppressing a magnetic resonance (MR) signal induced by fat among all MR signals, (1) a method employing a fat suppressing pulse (refer to, for example, “Magnetization Preparation during the Steady-state Fat-saturated 3D True FISP” written by Klauze Sheffler et al. (Magnetic Resonance in Medicine, 2001, Vol. 45, pp. 1075–1080), and (2) a method employing fluctuating equilibrium magnetic resonance (FEMR) (refer to, for example, “Fluctuating Equilibrium MRI” written by Shreyas S. Vasanawala et al. (Magnetic Resonance in Medicine, 1999, Vol. 42, pp. 876–883) are known.
However, when the distribution of static magnetic field strengths is not uniform, an image constructed in the method employing fat suppressing pulses or an image constructed according to the FEMR technique suffers from band artifact. Furthermore, fat suppression to be achieved using fat suppressing pulses disorders the steady state of a magnetic field, and is therefore unsuitable for the SSFP method.
U.S. Pat. No. 2,398,329 (Patent Document 1) has disclosed a technology of performing magnetic resonance imaging with spins in an object of imaging excited in the SSFP, and constructing a water image or a fat image on the basis of the sum or difference between echoes (MR signals) acquired using an RF pulse whose phase does not change and echoes acquired using an RF pulse whose phase is alternately changed between 0 radian and π radian.
According to the method, since echoes are acquired using RF pulses exhibiting two kinds of phases, a scan time is long and a signal processing time is long.
The phase of an MR signal is affected by the inhomogeneity in static magnetic field strength. Attempts have been made to compensate for the inhomogeneity in static magnetic field strength in terms of facilities, but have confronted limitations. Consequently, there is an increasing demand for separating water from fat so as to construct an image while recognizing but being unaffected by the inhomogeneity in static magnetic strength.
The frequency of an MR signal induced by fat is different from that of an MR signal induced by water due to chemical shifts. A technology of separating water from fat by utilizing a phase difference deriving from a difference between the frequencies has been proposed.
A Dixon imaging method is a technique of acquiring two image data items, which represent MR signals that are induced by water and fat respectively and that are in phase or out of phase with each other, constructing a water image using the sum of the two image data items, and constructing a fat image using the difference thereof.
However, since the Dixon method requires production of two image data items, a scan time increases. Moreover, a rate at which the phase of a signal induced by fat differs from that of a signal induced by water decreases along with a decrease in static magnetic field strength. In order to acquire MR signals that are induced by water and fat respectively, that are in phase with each other, and that each include a gradient echo, a long echo time TE is needed. Consequently, signal attenuation increases. This poses a problem in that the Dixon method cannot be adapted to a system offering a low magnetic field.
In efforts to solve the foregoing problems, Japanese Unexamined Patent Application Publication No. 2001-414 (U.S. Pat. No. 3,353,826, Patent Document 2) has disclosed a technology of separating water from fat by adapting a single quadrature fat/water imaging (SQFWI) method to echoes acquired according to a phase cycling SSFP method.
The technology is briefed in Thesis 1 written by M. Miyoshi et al. and entitled “SSFP Fat/Water Separation by Fourier Transfer Phase Cycling and the Single Quadrature Dixon Method” (Proc. Intl. Soc., Magnetic Resonance in Medicine, Vol. 11, 2003, pp. 981).
The technology described in Japanese Unexamined Patent Application Publication No. 2001-414 and Thesis 1 is such that: a plurality of scans is performed by applying RF pulses, which exhibit different phases, with spins excited in the SSFP in order to acquire echoes; the echoes are two-dimensional Fourier-transformed and then two-dimensional inverse-Fourier-transformed; the resultant signals are handled according to the SQFWI method in order to remove the adverse effect of the inhomogeneity in a static magnetic field; and eventually, data representing water is separated from data representing fat.
Thesis 2 written by B. A. Hargreaves et al. and entitled “Fat Suppressed Steady-state Free Precession Imaging using Phase Detection” (Proc. Intl. Soc., Magnetic Resonance in Medicine, Vol. 11, 2003, pp. 548) describes that when the SSFP method is implemented under the conditions that a repetition time TR equals an in-phase time and an echo time TE equals a half of the in-phase time which equals a half of the repetition time TR, rectangular-wave signals whose waveforms look like a graph of a function and whose resonant frequencies are equivalent to a quotient of 2 by TR are acquired. The thesis concludes that signals sharing a single phase can be acquired successfully.
However, according to the method described in Thesis 2, the phases of signals induced by water and fat respectively are π (radian) and are consistent with each other. A certain hypothesis is needed for separation of the signal induced by water from the signal induced by fat.
[Patent Document 1] Japanese Patent No. 2,398,329
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-414 (Japanese Patent No. 3,353,826)
The technology described in Japanese Unexamined Patent Application Publication No. 2001-414 and Thesis 1 has the drawback that the repetition time TR must equal the product of the in-phase time by 2/n and the echo time TE must equal the quotient of the in-phase time by n (where n denotes an integer equal to or larger than 3). Besides, the repetition time TR is so short that a system for realizing the technology will be large in scale and include special components. For example, when n equals 3, if a magnetic field strength is 0.7 T, the repetition time TR is 6.5 ms. If the magnetic field strength is 1.5 T, the repetition time TR is 3.1 ms. The system therefore becomes large in scale and includes special components.
Furthermore, since at least two scans must be performed, a scan time is long.
The method described in Thesis 2 stipulates as the contents of a pulse sequence database (PSD) that the repetition time TR equals the in-phase time and the echo time TE equals a half of the in-phase time. For example, when a magnetic field strength is 0.7 T, the repetition time TR is 9.8 ms. When the magnetic field strength is 1.5 T, the repetition time is 4.6 ms. The large-scale special system configuration described in Thesis 1 need not be adopted.
However, according to the method described in Thesis 2, signals induced by water and fat assume opposite signs (they are out of phase with each other by π (radian)). A certain hypothesis must be established in order to discriminate the signal induced by water from the signal induced by fat. For example, the phases of the water and fat signals are discriminated from each other using an overall histogram, or any other complex processing is needed. At this time, there is a possibility that the phases of the water and fat signals may be identified inversely and that incorrect judgment may be invited.
Furthermore, according to the method of Thesis 2, since the phases of the water and fat signals are opposite to each other, echo times required for the water and fat signals are different from each other accordingly. Consequently, the fat signal strength gets lower.
As mentioned above, the related arts have the room for improvement from the viewpoints of a shorter processing time and easy and accurate separation of water from fat.